The description of the tiltrotor is as simple as it has been frequently given: It is a hybrid aircraft that flies like a turboprop but lands and takes off like a helicopter. Hence, its appeal: Its versatility offers the best of both the fixed-wing and rotary-wing worlds.
Does the hybrid aspect of this technology create new challenges for the aircraft’s electronics? In terms of avionics, not much. Both helicopters and airplanes essentially fly in the same environment–though helicopters commonly do so at lower elevations. Both navigate and communicate to ground facilities the same way, and both share comparable instrumentation.
With the Bell/Agusta 609 tiltrotor, a simple, clean cockpit panel, with its three main, active matrix liquid crystal displays (AMLCDs), belies the aircraft’s overall complexity. Behind that panel exists a sophisticated, state-of-the-art avionics suite based on Rockwell Collins Pro Line 21 CNS technology.
And, yes, it does include several functions that are unique to tiltrotors. Perhaps most apparent to BA609 pilots is the variable airspeed range that Rockwell Collins incorporated in the primary flight display. Designed to protect against aircraft stall, the display includes the conventional green arc that rotates around the airspeed ball, but according to Jan Heeren, system project engineer at Rockwell Collins, it also incorporates an overlapping gray arc, which depicts rotation of the engine/proprotor nacelle. Together the two arcs provide a "management tool" that BA609 pilots will use to remain comfortably within the aircraft’s airspeed envelope, regardless the mode of flight, according to Don Barbour, Bell/Agusta Aerospace Co.’s executive marketing director.
Also unique to the BA609 tiltrotor are the various electrical power sources, offering multiple redundancy. The aircraft’s main power source comes from the generators on the BA609’s two Pratt & Whitney Canada PT6C-67C turboshaft engines. In addition, the aircraft has two additional generators on the driveshafts (also called "cross-shafts"–they are designed to keep both proprotors turning should one engine fail) in the wings. These are primarily for the BA609’s anti-icing system, but according to Barbour, their electrical supply can be rerouted to power other systems. Finally, the BA609 has a "basic ship battery," he adds, "which will keep all of the aircraft electronics on-board going for up to 20 minutes." Thus, the power supply on this nine-passenger tiltrotor can to be described as triple-redundant.
The driver in designing the BA609’s electrical systems, says Barbour, "is to fly 20 to 45 minutes in icing conditions without electrical de-icing because of a lightning strike." The possibility of a lightning strike during icing conditions is rare, he admits, "but we still need to do this to obtain transport category certification."
Among other things, the electrical system must power three flight control computers, each with separate power lines, to protect against power surges from lightning. The three British Aerospace Engineering (BAE) processors also manage the navigation and engine power in the BA609.
The folks at Bell/Agusta have planned another feature that is unique to tiltrotor technology. Specifically, it is an adjustment to the autopilot function in BA609’s flight control computers. "We look to couple [the autopilot] like on a fixed-wing aircraft," Barbour says, "but it will allow the pilots to make adjustments to manage a glideslope of up to 9�, as opposed to 1� to 3�, performed by a fixed-wing."
The tiltrotor can descend down a glideslope that is even steeper than a helicopter’s, because the nacelles can tilt back 5�, he says. "We’ve flown as steep as 15�, and 12� is certainly possible with the BA609," he adds. "There is such a wide range of approaches that can be made, and that’s why we’ve allowed for pilot adjustments."
Despite this very un-fixed-wing-like capability, Bell/Agusta wanted fixed-wing pilots to feel at home in the BA609. "The Pro Line 21 suite serves that," Barbour says.
Rockwell Collins introduced its Pro Line 21 CNS avionics in early 1999, some four years after it unveiled its first version of the Pro Line product line, which includes open systems architecture. The latest version, according to Ted Fuhrer, the company’s vice president and general manager of Business and Regional Systems, is to "perform all the voice and data communication services necessary as we transition to Free Flight." It is to prepare Rockwell Collins customers for the emerging communication, navigation, surveillance/air traffic management (CNS/ATM) environment, hence, the addition of the three letters, CNS, to the latest Pro Line 21 line-up.
The avionics package in the Bell/Agusta 609 includes the following:
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Dual VHF-4000 voice/data communications transceivers, which accommodate VHF data link (VDL) Mode 2 for high-speed data communication and support automated digital messaging, automatic flight-plan loading, and graphical weather depiction;
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The NAV-4000 navigation receiver, which includes dual VOR/ILS and ADF;
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GPS-4000A Global Positioning System receiver;
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Dual VOR/ILS (instrument landing system) by Rockwell Collins;
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DME-4000 distance measuring units;
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Dual RTU-4200 radio tuning units to control the integrated nav sensors, and the TDR-94D Mode S transponder with automatic dependent surveillance-broadcast (ADS-B) capability.
Optional equipment for the BA609–aimed at the corporate and deep-water offshore markets, among others–are:
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The TCAS-4000 traffic alert and collision avoidance system;
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The ALT-4000 radar altimeter;
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Solid-state weather radar;
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Flight director; and
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Rockwell Collins’ FMS-3000 flight management system.
The BA609’s cockpit panel appears quite symmetrical, with an 8-by-10-inch (20-by-25-cm) primary flight display before the pilot and the copilot and another, same-size display in the center for the engine instrument and crew alert system (EICAS). All three displays are vertical (portrait) in format and "highly reconfigurable," says Heeren. Not only can the display on one screen be shown on the other two, but also a "condensed EICAS" can be shown along with the primary display, so the pilot sees everything on one AMLCD, Heeren adds.
Between the three displays are two Rockwell Collins control display units (CDUs) for radio frequency tuning. In the panel’s center will be a standby attitude indicator, which also will show altitude and airspeed. And immediately below the center display, on the center console, is a multifunction display (MFD) for the flight management system.
Fully integrated (by simply adding a computer card) into the Pro Line 21 CNS suite will be a Smith Industries health and usage monitoring system (HUMS), incorporating vibration and trend analysis.
The BA609 avionics and flight controls incorporate ARINC 429 data bus architecture. The bus design uses simplex (one direction only) point-to-point bus lines to transmit and receive with both high speed (100 Kbps) and low speed (12 to 14 Kbps) data transmission rates. The 609 has more than 40 bus-controlled line replaceable units with over 125 individual buses.
Tedeco electric chip detectors from Easton Aerospace Controls will be fitted into the BA609’s proprotor and tiltrotor gearboxes. The BA609 tiltrotor has a Lear Astronautics triplex, digital fly-by-wire flight control system with Dowty Aerospace actuators. This system, according to Barbour, allows the pilot to fly the aircraft "intuitively," which means the control responses will "wash" seamlessly between aircraft and rotorcraft modes when, say, a roll maneuver is engaged.
First flight for the BA609 is scheduled for summer 2001. The aircraft will be fully equipped then, with the Pro Line avionics and BAE processors on board. Deliveries from Bell/Agusta Aerospace’s new Tiltrotor Assembly Center in Amarillo, Texas, are slated to begin in 2003, following the BA609’s certification, including for instrument flight. Customer training for the BA609 will be conducted at the Bell/Agusta headquarters, at Alliance Airport, Fort Worth, Texas.